1. Field of the Invention
The present invention relates to an apparatus for detecting a fault, i.e., leakage, in an exhaust system of an internal combustion engine.
2. Description of the Related Art
Three-way catalysts for simultaneously promoting the oxidation of incompletely burned hydrocarbons (HC) and carbon monoxide (CO) and the reduction of nitrogen oxides (NOx) formed by residual oxygen reacting with nitrogen in the air are used on internal combustion automotive engines to control exhaust emissions. To enhance the oxidation/reduction performance of the three-way catalyst, the air/fuel ratio (A/F), which indicates engine combustion state, must be controlled to within a very narrow range (called the window) centered on stoichiometry. To achieve this, in fuel injection control of an engine, an O2 sensor (oxygen sensor) for detecting whether the air/fuel ratio is richer or leaner than stoichiometry from the residual oxygen concentration in exhaust gas is mounted as an air/fuel ratio sensor, and feedback control of the air/fuel ratio is performed to correct the amount of fuel based on the sensor output.
In such an air/fuel ratio feedback control system, the O2 sensor for detecting the oxygen concentration is mounted as close as possible to the combustion chamber, that is, on the upstream side of the catalytic converter. There is also implemented a double O2 sensor system in which a second O2 sensor is mounted on the downstream side of the catalytic converter in order to compensate for variations in the output characteristic of the upstream O2 sensor. That is, on the downstream side of the catalytic converter, the exhaust gas is thoroughly stirred, and its oxygen concentration is almost in equilibrium by the action of the three-way catalyst; as a result, the output of the downstream O2 sensor changes little compared with the upstream O2 sensor, and thus indicates whether the air/fuel mixture as a whole is on the rich side or lean side. In the double O2 sensor system, sub air/fuel ratio feedback control is performed using the O2 sensor mounted downstream of the catalyst in addition to the main air/fuel ratio feedback control performed using the O2 sensor mounted upstream of the catalyst, and various constants used when calculating the air/fuel ratio correction coefficient in the main air/fuel ratio feedback control are corrected based on the output of the downstream O2 sensor, or a second air/fuel ratio correction coefficient based on the output of the downstream O2 sensor is introduced, thereby accommodating variations in the output characteristic of the upstream O2 sensor and improving the accuracy of the air/fuel ratio control.
In such a double O2 sensor system, if a fault such as an exhaust pipe crack occurs between the O2 sensor upstream of the catalyst and the O2 sensor downstream of the catalyst, outside air may be drawn into the exhaust pipe by exhaust pulsation, causing the downstream O2 sensor to erroneously detect that the exhaust air/fuel ratio is lean. If this happens, the engine air/fuel ratio is erroneously controlled toward the richer side, and the purification performance of the catalyst degrades, resulting in increased HC and CO emissions. There are also cases where the O2 sensor downstream of the catalyst is used to detect deterioration of the catalyst; in such cases, the above exhaust system fault can cause erroneous detection.
In view of this, Japanese Unexamined Patent Publication No. 8-210126 discloses a technique in which, when the main air/fuel ratio feedback control constant corrected based on the downstream O2 sensor output reaches a predetermined guard value, the catalyst deterioration determining process is inhibited by determining the situation as being an exhaust system fault (exhaust pipe leakage).
This prior art technique attempts to detect an exhaust system fault from a tendency to lean that the output of the downstream O2 sensor is showing, but the main air/fuel ratio feedback control constant corrected based on the downstream O2 sensor output is not a parameter that always corresponds one for one to an exhaust system fault, because the constant is also affected by other factors such as the deteriorating tendency of the catalyst. Therefore, with the above prior art, it is difficult to detect an exhaust system fault reliably and quickly.
Recent years have also seen the development of an internal combustion engine in which air/fuel ratio is controlled so that the three-way catalyst can always provide a constant and stable purification performance. That is, the three-way catalyst has an O2 storage capability and purifies the exhaust gas by adsorbing excessive oxygen when the exhaust gas is lean and by releasing necessary oxygen when the exhaust gas is rich, but such a capability is limited. To make effective use of the O2 storage capability, therefore, it is important that the amount of oxygen stored in the catalyst be maintained at a prescribed level (for example, one-half the maximum oxygen storage amount) so as to be able to respond to the next change in the air/fuel ratio of the exhaust gas, whether it is a change to a rich state or a lean state. When the amount of oxygen is maintained in this manner, a consistent O2 adsorption/desorption function can be achieved, ensuring a consistent oxidation/reduction performance of the catalyst.
In the internal combustion engine in which the O2 storage amount is controlled to a constant level to maintain the purification performance of the catalyst as described above, a wide-range air/fuel ratio sensor capable of linearly detecting air/fuel ratio is used, for example, and feedback control (F/B control) is performed based on proportional-integral operations (PI operations). In such an O2 storage amount constant control system, an O2 sensor may also be installed on the downstream side of the catalyst in order to compensate for variations in the output characteristic of the wide-range air/fuel ratio sensor. In this case also, there arise the same problems as those described for the double O2 sensor system.
It should also be recognized that the problem that, in the event of an automotive exhaust pipe fault, exhaust gases not purified by the catalyst are discharged into the atmosphere, or outside air is drawn into the exhaust pipe by exhaust pulsation, can also occur in an internal combustion engine equipped with only one air/fuel ratio sensor in the exhaust system.
The present invention has been devised in view of the above-outlined problems, and an object of the invention is to provide an apparatus that can reliably and quickly detect a fault in an exhaust system of an internal combustion engine.
To attain the above object, according to a first aspect of the present invention, there is provided an apparatus for detecting a fault in an exhaust system of an internal combustion engine, comprising: control means for providing a rich air/fuel ratio forcefully and continuously when detecting the presence or absence of a fault in the exhaust system; and determining means for determining that there is a fault in the exhaust system when a sensor mounted in the exhaust system does not produce an output that corresponds to the rich air/fuel ratio being provided under control of the control means. Since an exhaust system fault (leakage) is detected by forcefully enriching the air/fuel ratio, the apparatus of this invention can improve detection accuracy compared with an apparatus of the type that detects exhaust leakage during air/fuel ratio feedback control.
According to a second aspect of the present invention, there is provided an apparatus for detecting a fault in an exhaust system of an internal combustion engine, comprising: control means for providing a rich air/fuel ratio forcefully and continuously during engine idling when detecting the presence or absence of a fault in the exhaust system; and determining means for determining that there is a fault in the exhaust system when a sensor mounted in the exhaust system does not produce an output that corresponds to the rich air/fuel ratio being provided under control of the control means. As an exhaust system fault (leakage) is detected by forcefully enriching the air/fuel ratio when the engine is in an idling state in which exhaust pulsation is large, the apparatus of this invention achieves further enhanced detection accuracy.
According to a third aspect of the present invention, there is provided an apparatus for detecting a fault in an exhaust system of an internal combustion engine in which air/fuel ratio is feedback-controlled based on outputs of an upstream and a downstream air/fuel ratio sensor respectively mounted on the upstream and downstream sides of a catalytic converter installed in the exhaust system, comprising: enriching control means for stopping the feedback control and performing control to continuously maintain the air/fuel ratio of the engine in a rich condition; and fault determining means for determining that there is a fault in the exhaust system when the downstream air/fuel ratio sensor does not produce an output that corresponds to the rich air/fuel ratio being provided under control of the enriching control means.
According to a fourth aspect of the present invention, in the apparatus of the third aspect, preferably, the enriching control means performs the enriching control when the engine is in an idling state.
According to a fifth aspect of the present invention, in the apparatus of the third aspect, preferably, both the upstream air/fuel ratio sensor and the downstream air/fuel ratio sensor are O2 sensors for detecting whether the air/fuel ratio in exhaust gas is rich or lean, and the fault determining means determines the presence or absence of a fault in the exhaust system by comparing the outputs of the two sensors.
According to a sixth aspect of the present invention, in the apparatus of the third aspect, preferably the upstream air/fuel ratio sensor is a wide-range air/fuel ratio sensor whose output characteristic is substantially proportional to the air/fuel ratio in the exhaust gas, while the downstream air/fuel ratio sensor is an O2 sensor for detecting whether the air/fuel ratio in the exhaust gas is rich or lean, and the fault determining means determines that there is a fault in the exhaust system when the output of the downstream O2 sensor shows a value corresponding to a lean condition.
According to a seventh aspect of the present invention, there is provided an apparatus for detecting a fault in an exhaust system of an internal combustion engine equipped with means for stopping engine idling, comprising: control means for disabling the idling stop when a condition for detecting the presence or absence of a fault in the exhaust system occurs, and for operating the engine in an idling state with a rich air/fuel ratio; and determining means for determining that there is a fault in the exhaust system when a sensor mounted in the exhaust system does not produce an output that corresponds to the rich air/fuel ratio being provided under control of the control means. As earlier noted, it is preferable to detect an exhaust system fault (leakage) when the engine is in an idling state in which exhaust pulsation is large, but in a vehicle equipped with an idling stop function for energy conservation purposes, there is no idling region in the first place. In the apparatus of the seventh aspect, when detecting the presence or absence of a fault in the exhaust system, the engine is forcefully operated in an idling state so that the detection can be made.
According to an eighth aspect of the present invention, there is provided an apparatus for detecting a fault in an exhaust system of an internal combustion engine of a vehicle that uses the internal combustion engine and an electric motor as power sources, comprising: control means for operating the internal combustion engine with a rich air/fuel ratio when detecting the presence or absence of a fault in the exhaust system, by changing an operating ratio between the internal combustion engine and the electric motor so that the internal combustion engine operates within a designated operating range; and determining means for determining that there is a fault in the exhaust system when a sensor mounted in the exhaust system does not produce an output that corresponds to the rich air/fuel ratio being provided under control of the control means. In the case of a hybrid vehicle system, when detecting the presence or absence of a fault in the exhaust system by forcefully enriching the air/fuel ratio, the internal combustion engine is operated so that the exhaust pulsation becomes large, and the variations in the torque of the internal combustion engine are canceled out by the electric motor. This serves to improve the accuracy of detection of an exhaust system fault.
According to a ninth aspect of the present invention, when detecting the presence or absence of a fault in an exhaust system of an internal combustion engine equipped with a variable valve timing mechanism, the variable valve timing mechanism is controlled so that a negative pressure wave in the exhaust system becomes large. In the case of an engine equipped with a variable valve timing mechanism, the accuracy of detection of an exhaust system fault (leakage) can be enhanced by controlling the variable valve timing mechanism so that the negative pressure wave in the exhaust passage becomes large. Generally, when control is performed in such a direction as to increase the overlap period of the intake and exhaust valves, the negative pressure wave in the exhaust passage becomes large.